Pivoted-arm calipers are used extensively to measure the diameters of cylindrical and spherical objects. The caliper of U.S. Pat. No. 1,269,336 to Taylor is typical of pivoted-arm calipers which are presently available and includes a pair of curved arms which are rotatably connected at a pivot point. Each arm has a single free end or tip for contacting the surface of an object to be measured.
The first arm has a scale and the second arm has an indicating finger which moves relative to the scale as the angle between the arms changes. The scale is calibrated in such a manner that the position of the indicating finger with respect to the scale provides a measure of the straight-line distance between the free ends of the arms.
The caliper of U.S. Pat. No. 1,269,336 and other presently available pivoted-arm calipers determine the diameter of an object by direct measurement of the straight-line distance between two points on the surface of the object which are diametrically opposed, that is, two points which are opposite endpoints of a diameter. Such calipers use a two-point technique which requires that the free ends of the arms simultaneously contact two diametrically opposed points on the surface of the object.
The range of presently available pivoted-arm calipers is significantly limited by this requirement. When measuring the outside diameter of a spherical object, for instance, the largest diameter such calipers are capable of measuring is only approximately one and one-third times greater than the straight-line distance from the free end of a caliper arm to the pivot point.
Therefore, with presently available pivoted-arm calipers, the measurement of a circular object having a large diameter requires a correspondingly large caliper. For example, the measurement of a spherical tank with a 20 foot outside diameter presently requires a caliper with arms each having a minimum length of approximately 15 feet from its free end to the pivot point. The limited range of these calipers is a significant disadvantage as it necessitates the use of large, unwieldy calipers when measuring large objects.
Therefore, a need exists for a caliper which has an extended range and which can measure diameters which are much greater in length than the length of the arms of the caliper.
In addition to limiting the range of measurement, the requirement that the free ends of the caliper arms contact diametrically opposed points on the surface of the object also makes it impossible to use presently available pivoted-arm calipers to measure diameters in many commonly occurring situations. For example, access to the object to be measured is often limited to such an extent that the free ends of the caliper arms cannot be placed in contact with diametrically opposed points on the surface of the object. This is frequently the case with pipes, tanks, or other vessels which are partially buried or closely surrounded by other objects. In such situations, the diameter of the object cannot be measured using presently available pivoted-arm calipers.
In addition, many objects which have a radius of curvature, such as right-angle curved ducts, also have a cross section in which the curved portion is less than a half circle. As a result, such objects do not have diametrically opposed points on their surfaces for the free ends of the caliper arms to contact. While such objects do not actually have a diameter, an indication of what the diameter would be if the curved portion were completely circular would be useful as it would permit the radius of curvature to be easily calculated. However, since these objects lack diametrically opposed points, presently available pivoted-arm calipers are unable to provide an indication of diameter.
Accordingly, a need also exists for a caliper which is capable of measuring diameters without contacting diametrically opposed points on the surface of the object.